In general, electrochromic devices are referred to as devices that experience a change in color due to an electrochemical redox reaction caused by the application of an electric field, resulting in a change in light transmission characteristics. Typical electrochromic materials include tungsten oxides found by S. K. Deb in 1969. Thereafter, electrochromism of various organic/inorganic materials has been studied, followed by continuous development and research into the application of electrochromic devices comprising such materials in the field of smart window and display technology.
Electrochromic materials are classified into reduction-colored materials and oxidation-colored materials. Reduction-colored materials are those colored by the acquisition of electrons and typically include tungsten oxides, Meanwhile, oxidation-colored materials are those colored by the loss of electrons and typically include nickel oxides and cobalt oxides. Other electrochromic materials include inorganic metal oxides such as Ir(OH)x, MoO3, V2O5, TiO2, etc., conductive polymers such as PEDOT (poly-3,4-ethylenedioxy thiophene), polypyrrole, polyaniline, polyazulene, polythiophene, polypyridine, polyindole, polycarbazole, polyazine, polyquinone, etc., and organic electrochromic materials such as viologen, anthraquinone, phenocyazine, etc.
The above inorganic metal oxides generate a change in color when lithium ions or hydrogen ions present in an electrolyte are doped into the inorganic metal oxides. On the contrary, as depicted in the following formula 1, conductive polymers, for example, polyaniline shows a light yellow color when it is present in a completely reduced state, while showing a blue color when it is present in a state doped with anions by oxidation. Various colors can be realized depending on the kinds of such conductive polymer. [Formula 1]

In addition to the above-mentioned inorganic metal oxides and conductive polymers, organic electrochromic materials include viologen compounds such as 4,4′-dipyridinium salt represented by the following formula 2. A viologen compound has three types of oxidation states, i.e., v2+ (colorless), V+ (blue) and V0 (light yellow), each oxidation state showing a different color: [Formula 2]

Meanwhile, U.S. Pat. No. 5,441,827 (Graetzel et al.) discloses a device having high efficiency and high response rate, the device being manufactured by coating an electrochemically active organic viologen compound, as a single layer, onto the surface of a nanoporous thin film electrode obtained by sintering metal oxide nanoparticles. Additionally, the device uses a mixture of a lithium salt with an organic solvent such as γ-butyrolactone and propylene carbonate, as liquid electrolyte. However, the device using an organic solvent-containing liquid electrolyte has disadvantages in that quenching rate is low, residual images are present after quenching and that the organic materials may be decomposed easily during repeated developing/quenching cycles. Moreover, because the device uses an organic solvent-containing liquid electrolyte, it has additional disadvantages in that evaporation and exhaustion of the electrolyte may occur, the electrolyte may leak out from the device to cause an environmentally unfavorable problem, and that formation into thin films and film-shaped products is not allowed.
U.S. Pat. No. 5,827,602 (V. R. Koch et al.) discloses an ionic liquid electrolyte based on AlCl3-EMICI (aluminum chloride-1-ethyl-3-methylimidazolium chloride) including a strong Lewis acid. The ionic liquid such as AlCl3-EMICI has no vapor pressure, and thus can solve the problem of exhaustion and decomposition of electrolyte. However, it may emit toxic gases when exposed to a small amount of moisture and oxygen. Moreover, the ionic liquid is problematic in that it has high reactivity with organic/inorganic compounds added to the electrolyte in a small amount and that it is easily decomposed at a temperature of 150° C. or higher.
U.S. Pat. No. 6,667,825 (Wen Lu et al.) discloses an electrochromic device that uses a conductive polymer and an ionic liquid such as [BMIM][BF4] containing no Lewis acid, as electrode and electrolyte, respectively. Use of the ionic liquid containing no Lewis acid results in improvement of stability and lifespan of electrochromic devices. Additionally, it is possible to solve, at least in part, the problems with which organic solvent-based liquid electrolytes and ionic liquid electrolytes containing a Lewis acid are faced, i.e., the problems of residual images after quenching, decomposition of electrolytes or the like. However, because the electrochromic device according to U.S. Pat. No. 6,667,825 uses an ionic liquid as liquid electrolyte, it still has problems in that leakage of electrolyte may occur and that formation into thin films and film-shaped products is not allowed.
In order to complement such disadvantages of liquid electrolytes, polymer electrolytes have appeared recently. For example, Maroco-A.De Paoli discloses a polymer electrolyte formed by mixing an organic liquid compound with poly(epichlorohydrin-co-ethylene oxide) (see, Electrochimica Acta 46, 2001, 4243-4249). However, the above polymer electrolyte shows a significantly low conductivity of about 10−5 S/cm. Additionally, S. A. Agnihotry discloses a polymer electrolyte having a high ion conductivity of 10−3 S/cm at room temperature, the polymer electrolyte being formed by adding a small amount of PMMA (polymethyl methacrylate) polymer and fumed silica to an electrolyte formed of propylene carbonate containing 1M LiClO4 added thereto (see, Electrochimica Acta. 2004). However, because the above polymer electrolyte still uses an organic solvent as electrolyte, it is not possible to solve the problems of low quenching rate, residual images after quenching, decomposition and exhaustion of organic solvent-based electrolytes, or the like.